The boron neutron capture therapy (BNCT) in which neutrons are used has been attracting attention (Non-Patent Document 1), since it can remove a tumor at a deep part for which there is no cure by a routine surgical operation with regard to encephaloma, hepar, melanoma and the like. Examples of a method for generating neutrons used in this therapy include a method using a nuclear reactor and a method using an accelerator. As a simple and safe neutron-generating method, at present, a method using an accelerator has been attracting attention. In the apparatus, neutrons are generated by a method in which protons are gradually accelerated to be made into a proton beam and thereafter a neutron is made by a method in which the proton beam impinge on a metal-made or graphite-made lump called a target (Non-Patent Document 2). Moreover, such an accelerator-type neutron generator is expected to be utilized as an apparatus for nondestructively inspecting the soundness of steel frames in a bridge, and is expected to be utilized in the automobile industry, aircraft industry and space industry (Non-Patent Document 3).
In order to generate neutrons with such an accelerator, it is necessary to make the beam have a high intensity. When this high-intensity beam passes through a target, the target results in high temperature and the target is tend to deform by heat. A heat sink for cooling is assembled behind the target and a cooling water circulates in the heat sink for cooling to protect the target from heat generated by the beam. However, a limited portion of the target is periodically intensely heated by the beam, and it follows that the heating is repeated for a long time. As such, there is a fear that not only a target but also a heat sink for cooling are broken by a heat shock. As a metal usually used for the heat sink is radioactivated and obstructs beam, only specific ones can be used. For example, titanium (22 W/mK), vanadium (31 W/mK), palladium (72 W/mK), niobium (54 W/mK), tantalum (58 W/mK) and the like are usable, but these are low in thermal conductivity. As such the heat from a target does not conduct to the whole heat sink and the cooling efficiency is poor.
In reducing the thermal resistance, an thermal interface material (TIM: Thermal Interface Material) plays an important role. However, since the interior of an accelerator is kept in an ultra-high vacuum state (10−6 to 10−7 Pa), in the case where heat release grease or a phase change sheet, which is generally used, is adopted, contamination of the interior of the apparatus is caused by outgassing. Moreover, when a TIM containing a metal and inorganic filler is adopted, there is a fear that filler scatters and contaminates the inside of a beam line.